1. Field of the Invention
The present invention relates to a bellows device having a plurality of recesses and ridges in the direction of expansion and contraction. More particularly, it relates to such a bellows device used for a pressure container such as a vacuum switch tube to operate depending on pressure difference between the interior and exterior of the pressure container.
2. Discussion of the Background
In general, a vacuum switch tube is so constructed as shown in FIG. 1 that openings formed at both sides of a pressure container 1 of an insulating material are closed by a stationary end plate 2 and a movable end plate 3; the interior of the thus closed pressure container is evacuated to render it in a vacuum condition; a stationary rod 6 is passed through the stationary end plate 2 and fixed to it, while a movable rod 7 is passed through the movable end plate 3 so as to be movable in its axial direction by means of an operating mechanism (not shown); a pair of electrodes 4, 5 are respectively attached to the opposed ends of the stationary and movable rods 6, 7 whereby the electrode 5 attached to the movable rod is connected to and disconnected from the electrode 4 depending on the movement of the movable rod 7. In this case, a bellows 8 is provided between the movable end plate 3 and the movable rod 7 so that the vacuum container is sealed in a vacuum condition and the movable rod 7 is permitted to move in its axial direction. A reference numeral 9 designates shielding members provided to surround the electrodes 4 and 5 and the stationary and movable rods 6 and 7.
There has so far been known to use a bellows device as shown in FIG. 2 which is an enlarged view of a part of the vacuum switch tube as in FIG. 1. In FIG. 2, the pressure container 1 contains a gas or a gas mixture subjected to pressure reduction to the extent of a high vacuum. The movable end plate 3 having an central opening is fixed to the lower end of the pressure container 1 and the movable rod 7 is passed through the central opening of the movable end plate 3 to be movable in its axial direction by means of the operating mechanism. There is provided the bellows 8 whose one end is firmly connected to the movable rod 7 and the other end is firmly connected to the movable end plate 3, the bellows being made of a thin-walled metallic tube and having a plurality of recesses 8b and ridges 8a in the direction of expansion and contraction. An intermediate fitting 10 is interposed at the middle portion of the bellows 8 whereby it can slide on the outer circumferential surface of the movable rod 7 when the rod 7 is caused to move in its axial direction.
There have been known a bellows valve, a vacuum switch and so on as a device which utilizes a bellows device to keep air-tightness of a container and to be capable of moving in its axial direction. In such devices, it is necessary to reduce stress for one ridge 8a of a bellows device so that the bellows device having a long stroke and being usable for a long term can be obtained. Accordingly, it necessarily requires a bellows device having a number of ridges 8a and as a result, a ratio of the length to the diameter of the bellows device generally exceeds 1.5. However, preparation of a bellows device having a large L/D ratio is very difficult. Accordingly, plural pieces of bellows have been connected in a multi-stage to obtain a desired performance.
Use of a bellows device is variable depending on circumstances such as gas or liquid used, pressure and so forth. Generally, a bellows device is operated by a pressure difference between the interior and the exterior of the bellows devices. In this case, when a bellows device having a large L/D ratio is used, there takes place deformation of the bellows 8 into a generally L-shape at the middle portion, in other words, there takes place so-called buckling phenomenon. To avoid such drawback, an intermediate fitting 10 is provided at a position susceptible to cause buckling phenomenon in the bellows 8.
Thus, the buckling phenomenon occurs in the bellows device having a large L/D ratio when a pressure difference takes place between the interior and the exterior of the bellows device. In this case, if the intermediate fitting 10 to prevent the buckling is not provided, there is excessive stress at a position in the bellows where buckling takes place. As a result, movement of the bellows device in the axial direction hinders a prolong term of use of it even though the number of ridges 8a is increased to reduce the amplitude of vibration per one ridge. Even when the intermediate fitting 10 is used to prevent the buckling, there is caused disadvantages such that deformation due to wearing takes place in the bellows device between the movable rod 7 and the intermediate fitting 10 for prevention of buckling or the movable rod 7 is shaved by the intermediate fitting 10 to hinder smooth movement of the movable rod 7 or to cause sticking due to friction. Further, when the movable rod 7 is operated quickly, there is delay in the movement of the intermediate fitting 10 for prevention of buckling at the time of initiation and stoppage of the movable rod 7 whereby an excessive force is applied to the bellows 8 thereby to shorten the life time of the device.